1. Field of the Invention
The present invention relates to a circuit module in which a sheet embedded with electronic components and provided with wiring patterns is folded into layers connected with each other, and also relates to a method for manufacturing such a circuit module.
2. Background Art
Mobile devices have been increasing in functionality while reducing in size and weight. With this tendency, circuit modules having a circuit board and electronic components mounted on the circuit board have been expected to be higher in performance and density. As smart cards and memory cards spread, there is a growing need for flexible circuit modules that are thin, high-performing, highly-reliable and inexpensive.
Japanese Patent Unexamined Publication No. 2002-43513 discloses a semiconductor device having a multilayer structure shown in FIG. 11 and a method for manufacturing the semiconductor device. According to this manufacturing method, as shown in FIG. 11, a plurality of semiconductor chips 1010 and components 1020 are mounted on support 1030 provided with wiring patterns. Later, insulating resin 1040 is applied to mold. Support 1030 provided with these components is folded in units of regions to form three stacked layers, and is then molded with insulating resin 1050. This is how the semiconductor device with the multilayer structure is obtained.
In the case of this device, in order to maintain the positional relation between semiconductor chips 1010 and components 1020 until the final condition, support 1030 is required to have appropriate stiffness. This sets a limit to reducing the thickness of modules having such a structure.
Japanese Patent Unexamined Publication No. H02-239695 discloses a multilayer printed wiring board, which also has a multilayer structure. According to the disclosure, first of all, a two-sided flexible substrate is prepared by forming a flexible sheet provided with necessary wiring patters and electrode pads. Then, this flexible substrate is folded at appropriate positions into stacked layers. The exposed regions of the conductors stacked with each other are bonded by applying solder or conductive adhesive so as to form a multilayer structure. This results in the multilayer printed wiring board. In this method, however, the printed wiring board is not embedded with electronic components, thereby making it difficult to manufacture a high-performing electronic circuit.
U.S. Pat. No. 6,225,688 discloses a mounting structure in which a film substrate is mounted with electronic components with their faces down, and is folded into layers.
Japanese Patent Unexamined Publication No. 2001-93934 discloses a method for mounting a semiconductor element. In this method, bumps are formed on a semiconductor element by wire bonding. The semiconductor element is embedded in a thermoplastic resin, with the bumps exposed. Then, wiring patterns are formed on the surface of the thermoplastic resin having the exposed bumps. Later, more thermoplastic resin is applied to mold. Thus, this patent document discloses a method for mounting a semiconductor element, but not a multilayer structure.
For mobile devices of various kinds, smart cards, memory cards and the like, circuit modules in which a flexible sheet is mounted with semiconductor chips and components and also provided on both sides thereof with wiring patterns are widely used. For the purpose of increasing the mounting density, circuit modules in which a substrate mounted with semiconductor chips and components is folded into a lager number of layers have been being developed.
However, in smart cards, memory cards and the like, it is required not only to improve the mounting density and reliability, but also to achieve high functionality in a shape that is standardized as the product shape.
In recent years, circuit scale (such as memory capacity) has been growing with the development of mobile devices, smart cards and memory cards. In order to meet this demand, in the aforementioned conventional examples, a flexible sheet mounted with electronic components is folded to form a multilayer substrate or a circuit module with layers stacked via an adhesive layer. Such a multilayer structure can increase the circuit scale and the like. However, the difference in material between the adhesive and the flexible sheet makes them have different thermal expansion coefficients from each other. The difference in the thermal expansion coefficients produces a stress which is likely to cause delamination. Preventing a decrease in reliability requires a substrate having a certain thickness; however, in smart cards and the like which are expected to have a limited thickness, it is difficult to expand the circuit scale by folding the substrate into stacked layers.
To solve the aforementioned problems, the present invention has an object of providing a circuit module in which a flexible sheet embedded with semiconductor chips and components is folded at predetermined positions into layers that are thermal-bonded and integrated with each other by applying a heat and pressure treatment, and also providing a method for manufacturing such a circuit module.